The present invention relates generally to apparatus and methods for testing selected characteristics of viscoelastic fluids such as lithographic printer's ink and varnish. More particularly the invention relates to such methods and apparatus that measures the force coupled by an ink or fluid between two members that create a "nip" as they move relative to each other. More specifically, the apparatus of the present invention simulates the nip created by two rollers in a printing press. The apparatus is capable of making such measurements at various levels of water concentration and various relative speeds.
In the lithographic printing industry, it is important to understand that one does not print with "ink" but instead prints with an "ink-water emulsion". This "ink-water emulsion" is formed on the lithographic printing press when the lithographic printing ink and fountain solution (water with various additives) go through numerous roller nips on a printing unit(s) on a lithographic printing press. Inks in the lithographic printing industry are either considered "lithographically stable" or "lithographically unstable". A "lithographically stable" ink or varnish is an ink or varnish which exhibits little or no change in its high shear rheology (structure) as a result of variations in the amount of water emulsified into the ink or varnish as it passes through the numerous roller nips on a lithographic printing press. A "lithographically unstable" ink or varnish is an ink or varnish which exhibits significant changes in its high shear rheology as a direct result of variations in the amount of water emulsified into the ink or varnish as it passes through the numerous roller nips on a lithographic printing press. In the past it was generally believed in the lithographic printing industry that to print properly an ink or varnish must absorb a particular percentage of water (fountain solution). However, it is now understood that it is more important to know how ink's or varnish's high shear rheology changes as a result of the water (fountain solution) emulsification rather than simply how much water or fountain solution the ink will absorb. To determine the rheological values of a printing ink, most quality assurance labs use an inkometer to measure tack and misting, a "flow plate" to measure ink fountain flow behavior, a viscometer such as a "drop rod" viscometer or preferably the Duke Custom Systems, Inc. D-2000 viscometer to measure viscosity and stress at rates of shear from 2.5 sec-1 to over 7,500 sec-1, a flow plate to measure ink fountain flow behavior, and a Duke D-10 ink water emulsification tester to measure the water pick-up of inks.
The Duke D-2000 discussed in U.S. Pat. No. 5,142,900 issued to the present inventor describes a substantially automated viscometer. The D-2000 includes a series of high-shear rod, and collar viscometer elements that are designed to eliminate the problems associated with drop rod viscometers, and to provide accurate and reproducible rheological test data on highly structured viscoelastic printing inks, varnishes, resin solutions and flushes at higher and lower shear rates than possible with a drop rod viscometer.
The D-10 tester described in the U.S. Pat. No. 4,403,867 patent by Horace Duke was designed to provide empirical data for lab testing purposes. Some of the additives used to adjust ink or varnish based on the test data obtained from the D-10 have a detrimental effect on actual press performance. The D-10 was not designed to predict the stability of an ink.backslash.water emulsion at press speeds. None the less, it is not uncommon for an ink to be within specifications for various laboratory tests and still exhibit undesirable, unstable behavior on the printing press. When a printer or ink manufacture's lab is unable to detect this instability before the ink or varnish is used on a press, longer startup time, increased waste, reduced press speeds, and higher color variations are the result. These problems, of course, have a detrimental effect on a printer's efficiency and profitability.
The present invention, however, improves over the D-10 tester in that it allows a lab technician to quickly and easily make an ink/water emulsion that exhibits the characteristics of an emulsion formed on a high-speed press. It makes an emulsion of ink and water or fountain solution at velocities that range from as low as 100 LFM (linear feet per minute) to more than 3,000 LFM by emulating the different energy levels an ink experiences on the roller train(s) of lithographic printing press(es) passing through the roller nip. Press correlation tests have shown that this energy-input simulation is very important in generating emulsions with characteristics that mirror those found on high performance presses.